The Electric Power Research Institute has predicted a shortage of electricity in the future. Spurred by the predicted energy crises and the prospect of a large capital investment necessary to meet customer peak load requirements, more and more electric utility companies are offering reduced rates for "off-peak" power, when the demand for electricity is much lower. To take advantage of these off-peak rates, electric storage heaters have been developed that convert electrical energy, during off-peak hours, into heat. The heat generated is stored internally in the heater for circulation by thermostatically controlled fans during peak rate time periods.
The Control Electric Corp. of Burlington, Vt., sells an electric storage heater for home use which consists of a resistance heating wire surrounded by an air input-output passage which in turn is surrounded by a ceramic brick heat storage core. The electrical energy flowing through the wire heats the wire. The heat from the wire is transferred to the air which in turn transfers the heat to the ceramic bricks. The heat transfer efficiency from the wire to the air, from the air to the ceramic bricks and again from the bricks to the air is poor.
Heat transfer efficiency of electric storage heaters could be improved (1) if the ceramic core contacted by the wire, and (2) if the ceramic core had a large surface area (much larger than the area contacted by the wire). One proposal to increase the heat transfer efficiency is to wind the heating wire around a circular ceramic pipe by providing spiral grooves in the ceramic. However, this proposal suffers several disadvantages. One disadvantage is that the cutting of spiral grooves around the outside of the ceramic involves a sophisticated operation because the ceramic pipe must be rotated and moved lengthwise at precisely the right speed so that the grooves will connect. Furthermore, if the pipe is cut while it is soft (before it if fired), the pipe tends to deform as it is pushed and turned. On the other hand if the pipe is cut after it is fired, cutting is difficult and may require expensive diamond cutting tools. A second disadvantage is that a circular pipe of ceramic tends to deform during its manufacture, (1) when the ceramic pipe is extruded horizontally onto a moving belt because the soft pipe contacts the belt at only one point, and (2) when the soft pipe is cut into individual heat storage cores. When grooves are cut in a deformed pipe, the grooves will be deeper in some pipe portions increasing the chance of core breakage.